Understanding the Standard Penetration Test (SPT)

The Standard Penetration Test (SPT) is a common in-situ test used in geotechnical engineering to estimate the relative density and angle of shearing resistance of granular soils, and the consistency of cohesive soils. A standard split-spoon sampler is driven into the ground by blows from a standard hammer.

The SPT N-Value and Penetration Depth

The test involves driving the sampler a total distance of 450 mm (18 inches) below the bottom of the borehole or footing level. This driving is done in two stages:

• An initial seating drive of 150 mm (6 inches). The number of blows for this seating drive is usually recorded but not used to calculate the N value.
• A subsequent test drive of 300 mm (12 inches). This 300 mm is usually divided into two consecutive 150 mm intervals.

The Standard Penetration Number (N) is defined as the number of blows required to achieve the last 300 mm of penetration.

For example, if the blow counts for the three consecutive 150 mm intervals are $N_1$, $N_2$, and $N_3$ respectively (where the first 150 mm is the seating drive), the N value is calculated as:

\( N = N_2 + N_3 \)

This means the N value represents the blow count for the second and third 150 mm segments, which together constitute a total penetration of 300 mm.

Therefore, the standard penetration number (N) specifically refers to the number of blows required for 300 mm penetration, which is the test drive portion after the initial seating drive.